There is a move in the automotive industry to distributorless ignition systems of one coil per spark plug, and particularly towards plug-mounted coils. Motivations for this are more compact ignition, elimination of electromagnetic interference, and higher ignition efficiency (no distributor or spark plug wires), as well as other reasons.
There is also a desire to maintain and even raise, the spark plug energy that is delivered to the combustible mixture for ignition. While energy delivery efficiency of plug-mounted coils increases due to elimination of the distributor and spark plug wires, the constraints on the coil size reduce the energy that can be stored in the core and delivered to the spark gap. The coil winding resistance increases as the coil diameter is reduced in inverse relationship to the fourth power of the diameter, to make the coil ever less efficient as it is made smaller. The high coil primary inductance Lp of 2 to 8 milliHenry (mH), and low peak primary current Ipo of typically 4 to 10 amps available from a car battery of voltage Vb (of 6 to 13 volts), limit the energy that can be stored and delivered to the spark gap and limit the magnitude and quality of the spark that is delivered (50 milliamps typical spark current).
There is a need for an improved ignition with coils that are compact, light weight, inexpensive, and simple to fabricate and are suitable for plug mounting (or locating near the plug) which can store high energy of 150 to 600 millijoules (mj) and deliver high spark energy of 120 to 500 mj with high energy delivery efficiency. There is also a need to improve the overall operation of the inductive ignition system to permit higher switch break currents and higher stored energy while placing less stress on the coil's magnetic core and power switch.